The present invention relates to a dielectric porcelain composition for laminated ceramic capacitor, and more particularly to a dielectric porcelain composition which is sintered at a low temperature of not more than 1100.degree. C. as well as which has a good electrostatic capacity over temperature and a high dielectric constant.
Dielectric porcelain compositions containing a barium titanate as a main component, namely barium titanate base compounds are widely used for laminated ceramic capacitors. Particularly, barium titanate base compounds are widely used as dielectric porcelain compositions for laminated ceramic capacitors which have Y5V properties (-30.degree. C.-85.degree. C.: .DELTA.C/C (20.degree. C.): +22%--82%) of EIA standard and X7R properties (-55.degree. C.-125.degree. C.: .DELTA.C/C (20.degree. C.): +15%--15%) of EIA standard. The barium titanate base compounds have a high sintering temperature of not less than 1300.degree. C. For this reason, it is necessary to use expensive platinum or palladium for an internal electrode, resulting in a high cost of the capacitor. It has not been reported that the barium titanate base compounds may be used as dielectric porcelain compositions for laminated ceramic capacitors which have Y5U properties and Y5T properties between Y5V properties and X7R properties.
To settle the above problem with the high cost, lead based perovskite compounds have widely been studied as dielectric porcelain compositions for laminated ceramic capacitors and have been partially made practical. The lead based perovskite compounds have a low sintering temperature of not more than 1100.degree. C. For this reason, it is possible to use silver palladium which is inexpensive for the internal electrode of the laminated ceramic capacitor. It is further possible to combine a plurality of the lead based perovskite compounds for allowing variable design choices to obtain required various properties. For example, it is possible to prepare a dielectric porcelain composition having a high dielectric constant of not less than 2000, or other dielectric porcelain compositions of X7R properties in EIA standard, which shows a small variation of the dielectric constant over temperature. The former dielectric porcelain composition may be a ternary complex system of lead based perovskite compounds, for example, lead magnesium niobate (Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 : PMN), lead titanate PbTiO.sub.3 : PT) and lead nickel niobate (Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3 : PNN). This ternary lead based perovskite compounds is disclosed in the Japanese Laid-open Patent Application No. 48-81097. The latter dielectric porcelain compositions are disclosed in the Japanese laid-open Patent applications Nos. 58-60671, 60-42277, and 60-36371, wherein there is used a ternary complex system of lead based perovskite compounds, for example, lead magnesium tungstate (Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 : PMW), lead titanate (PbTiO.sub.3 : PT) and lead zirconate (PbZrO.sub.3 : PZ). In this system, an additive is used for improvement in the electro-static capacity over temperature.
In the Japanese laid-open Patent application No. 58-60671, there is disclosed a lead based perovskite compound showing a small variation within .+-.20% in capacity between different temperatures, for example, 30.degree. C. and 85.degree. C. However, dielectric constant of this lead based perovskite compound is not more than 5000 at a room temperature. In order to improve the capacitance properties over temperature, it is effective to add manganese compounds to the above lead based perovskite compound. The addition of the manganese compounds, however, causes a considerable reduction of the dielectric constant of the lead based perovskite compound. Another method for improving the capacitance properties over temperature is to replace partially lead contained in the lead based perovskite compound with other rear earth metals such as barium, strontium and calcium.
The Japanese laid-open patent application No. 1-298061 discloses that leads contained in the lead based perovskite compounds of the ternary complex system, for example, lead magnesium niobate (Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 : PMN), lead titanate (PbTiO.sub.3 : PT) and lead zirconate (PbZrO.sub.3 : PZ) are partially replaced with at least one of barium and strontium. In this case, it is difficult to use inexpensive silver palladium for an internal electrode of the capacitor since the sintering temperature is relatively high in the range of 1000.degree. C. to 1300.degree. C.
The Japanese laid-open patent application No. 4-115408 discloses that leads contained in the binary complex system of lead magnesium niobate (Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 : PMN) and other lead based perovskite compound are partially replaced with at least one of barium, strontium, calcium and silver. In this case, it is difficult to control the variation of the dielectric constant within .+-.20% between different temperatures of -30.degree. C. to +85.degree. C.
The Japanese laid-open patent application No. 63-221506 discloses that leads contained in lead magnesium tungstate (Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3 : PMW), lead titanate (PbTiO.sub.3 : PT) and lead zirconate (PbZrO.sub.3 : PZ) are partially replaced with at least one of barium, strontium and calcium. Further, either magnesium oxide or zinc oxide and manganese oxide are added to ensure a relatively high dielectric constant and also satisfy B-properties of JIS standard.
As described above, in the prior art, it would be difficult to obtain a composition which has a high dielectric constant at a room temperature, wherein the high dielectric constant shows a small variation over various temperatures as well as which has a sintering temperature of not more than 1100.degree. C.
In recent years, requirements for scaling down of and increasing capacitance of the laminated ceramic capacitor have been on the increase. To obtain the scaling down of the capacitor, it is required that each of the laminated ceramic layers constituting the capacitor has a small thickness, for example, not more than 10 micrometers. For this reason, it is required that the diameter of particles of a dielectric porcelain composition sintered would be within 2-3 micrometers.
If a lead ion-site of the lead based perovskite compound is replaced with other rare earth metals such as barium, strontium and calcium, Curie point is largely shifted to a low temperature, thereby resulting in a difficulty in material design.